Athermalization and pressure desensitization of diffraction grating based spectrometer devices

ABSTRACT

A device for monitoring wavelength division multiplexed optical signals for use in an optical network and in an optical performance monitor. A device has a structure for supporting components of the device. An optical component is supported at one end of the structure for transmitting the optical signals. A diffraction grating is supported at an opposing end of the structure for diffracting the optical signals from the optical component. An optical sensor is supported in relation to the diffraction grating by the structure for monitoring the optical signals. A telephoto lens assembly is supported by the structure and disposed between the optical sensor and the diffraction grating, the lens assembly having a focal length for focusing the optical signals in relation to the optical sensor. Thermal effects on the structure are balanced against thermal effects on the lens assembly. A prism is disposed between the lens assembly and diffraction grating. The prism is configured to anamorphically compress the diffracted optical signals. Thermal effects on the diffraction grating are balanced against thermal effects on the lens and prism.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is related to and claims priority from U.S.Provisional Patent Application No. 60/208,477, filed Jun. 2, 2000, whichis incorporated by reference herein in its entirety. This application isrelated to and claims priority from U.S. Provisional Patent ApplicationNo. 60/208,478, filed Jun. 2, 2000 which is incorporated by referenceherein in its entirety. This application incorporates by reference U.S.Patent Application entitled Device and Method for Optical PerformanceMonitoring in an Optical Communications Network filed Nov. 28, 2000.This application incorporates by reference U.S. Patent Applicationentitled Optical Performance Monitor with Optimized Focus Spot Sizefiled Nov. 28, 2000.

FIELD OF THE INVENTION

The present invention relates generally to wavelength divisionmultiplexed optical signals, and more particularly, to minimizing thethermal and pressure effects on diffraction grating based spectrometersystems incorporating wavelength division demultiplexing devices.

BACKGROUND OF THE INVENTION

The telecommunications industry has grown significantly in recent yearsdue to developments in technology, including the Internet, e-mail,cellular telephones, and fax machines. These technologies have becomeaffordable to the average consumer such that the volume of traffic ontelecommunications networks has grown significantly. Furthermore, as theInternet has evolved, more sophisticated applications have increaseddata volume being communicated across telecommunications networks.

To accommodate the increased data volume, the telecommunications networkinfrastructure has been evolving to increase the bandwidth of thetelecommunications network. Fiber optic networks that carry wavelengthdivision multiplexed optical signals or channels provide forsignificantly increased data channels for the high volume of traffic.The wavelength division multiplexed optical channels or polychromaticoptical signals comprises monochromatic optical signals. The wavelengthdivision multiplexed optical channels carry time division multiplexeddata containing information, including voice and data. Contemporaryoptical networks can include forty or more monochromatic opticalchannels on a single fiber and each monochromatic optical channel cancarry many thousands of simultaneous telephone conversations or datatransmissions, for example.

An important component of the fiber optic networks is an opticalperformance monitor (OPM) for monitoring the performance of the opticalsystem. The OPM provides a system operator the ability to monitor theperformance of the individual substantially monochromatic opticalsignals. The optical performance monitor may measure the followingmetrics: power level, center wavelength, optical signal-to-noise ration(OSNR), interference between channels such as crosstalk, and laserdrift. By monitoring these metrics, the optical network operator canidentify and correct problems in the optical network.

The OPM may include a dispersion engine and an optical sensor. Thedispersion engine may include lenses and a dispersion device, such as adiffraction grating. The lenses process the polychromatic optical signaland cause the polychromatic optical signal to be incident to thedispersion device at a near-Littrow condition, which is a conditionwhere the angle of the incident light beam is reflected back toward thesource of the incident light beam near the incident angle at at leastone wavelength. The dispersion device diffracts the polychromaticoptical signal into its component substantially monochromatic opticalsignals, which are diffracted at angles as a function of the wavelengthof each substantially monochromatic optical signal. Each substantiallymonochromatic optical signal forms a spot that is focused at distinctlocations along the optical sensor.

Both the mechanical and optical components of the spectrometer areaffected by changes in temperature. They expand and contract changing inrelative position, and also changing in optical properties.Additionally, changes in pressure cause changes in optical properties ofair within the spectrometer. These changes must be calibrated out orthey will affect the quality of the information received from thespectrometer. Thus, it is desirable to minimize the effects oftemperature and pressure on the spectrometer.

SUMMARY OF THE INVENTION

To overcome the adverse affects of changes in temperature and pressure adevice for monitoring wavelength divisions multiplexed optical signalshas been athermalized and desensitized to pressure. The device can alsobe part of an optical network. The device has a structure for supportingcomponents of the device. An optical component is supported at one endof the structure for transmitting the optical signals. A diffractiongrating is supported at an opposing end of the structure for diffractingthe optical signals from the optical component. An optical sensor issupported in relation to the diffraction grating by the structure formonitoring the optical signals. A lens assembly is supported by thestructure and disposed between the optical sensor and the diffractiongrating. The lens assembly has a focal length for focusing the opticalsignals in relation to the optical sensor. The diffraction grating hasan angular dispersion that changes with temperature and the focal lengthchanges with temperature. The product of the focal length and angulardispersion remains substantially constant with temperature. Optionally,this can be calibrated with software and a temperature sensing system.

The spectrometer further includes a prism supported by the structure anddisposed between the lens assembly and diffraction grating. The prismhas an angular dispersion that changes with temperature. The product ofthe focal length and the sum of the angular dispersion of the prism andthe angular dispersion of the grating remains substantially constantwith temperature. The change in index of refraction with temperature ofthe prism is a value approximately equal to the negative value of thecoefficient of thermal expansion of the diffraction grating. A change inindex of refraction with temperature of the prism is substantiallywithin 30% of a negative value of a coefficient of thermal expansion ofthe diffraction grating. The prism is configured to anamorphicallycompress the diffracted optical signals. A first prismatic region formedbetween the prism and the lens assembly is opposed to a second prismaticregion formed between the prism and the diffraction grating. The firstprismatic region has a first angle measured between the lens assemblyand the prism and the second prismatic region has a second anglemeasured between the prism and the diffraction grating, the second anglebeing approximately equal to the first angle.

In an embodiment without a prism, the coefficient of thermal expansionof the diffraction grating is a value chosen to be approximately equalto a negative of the change in index of refraction with temperature ofair. In this case, the diffraction grating has a coefficient of thermalexpansion of approximately 0.5 PPM/degree Celsius to 1.5 PPM/degreeCelsius.

The lens assembly is constructed of a material chosen to minimize itsvariance in focal length over temperature. The assembly comprises atelephoto lens. A coefficient of thermal expansion of the structure anda change in index of refraction with temperature of the lens assemblyare values selected so that a length of the structure changessubstantially proportionally with the focal length of the lens assemblyin response to temperature changes, whereby the lens assembly remainssubstantially focused in relation to the optical sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the system and method of the presentinvention may be obtained by reference to the following DetailedDescription when taken in conjunction with the accompanying Drawingswherein:

FIG. 1 is a block diagram of an optical performance monitoring system;

FIG. 2 is a side elevational view of a spectrometer device according toan embodiment of the present invention;

FIG. 3 is a perspective view of a portion of the spectrometer device ofFIG. 2;

FIG. 4 is an end view of the portion of the spectrometer device of FIG.2;

FIG. 5 illustrates a general construction of a diffraction gratingassembly;

FIGS. 6A-6B illustrate multiplexing and demultiplexing functions of awave division multiplexing/demultiplexing device;

FIG. 7 illustrates a pass band and central wavelength of a monochromaticbeam;

FIG. 8 illustrates the geometry of a prism; and

FIG. 9 is a block diagram of an optical communications system accordingto an embodiment of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings in which a preferred embodimentof the invention is shown.

Optical networks are utilized to handle telecommunications trafficcaused in part by the Internet, mobile communications, and facsimilecommunications. To increase the bandwidth of optical networks, multiplechannels are multiplexed into a single fiber optic line throughwavelength division multiplexing. A wavelength divisionmultiplexer/demultiplexer (WDM) is utilized to join a multiple number ofsubstantially monochromatic optical signals into a polychromatic opticalsignal in the multiplexing case, and separate a polychromatic opticalsignal into a multiple number of substantially monochromatic opticalsignals in the demultiplexing case. A monochromatic optical signal isdefined as being a narrowband optical signal. Characteristics, such aswavelength and signal power, and signal to noise ratio of eachmultiplexed optical signal in a polychromatic line are monitored with anoptical performance monitor (OPM) or spectrometer.

FIG. 1 is a block diagram of a system 500 having a spectrometer 505 usedto measure and display power of substantially monochromatic opticalsignals as combined into a polychromatic optical signal travelingthrough a fiber optic line 525. An optical beam splitter 530 is used toextract a percentage of the polychromatic optical signal from the fiberoptic line 525 and direct the extracted polychromatic optical signal tothe spectrometer 505. Spectrometer 505 operates to demultiplex thepolychromatic optical signal into its constituent substantiallymonochromatic signals and monitoring each, as is described below withreference to FIGS. 6A and 6B.

Referring to FIG. 2, there is shown a side view of a preferredembodiment of a spectrometer 10 in accordance with the presentinvention. The spectrometer 10 comprises a plurality of optical sensors14, a collimating/focusing lens 16 assembly, a prism 17, reflectivediffraction grating assembly 11, a coupling component 20, and acorresponding input optical fiber 22. All of the above-identifiedcomponents of the spectrometer 10 are disposed along an optical axis X—Xof the spectrometer 10, as will be described in more detail below.

Optical sensors 14 are sensors used for monitoring characteristics -ofthe optical signal, and may be any device, for example a photo-diode,capable of monitoring the desired characteristics. Sensors 14 aregrouped into a one-dimensional sensor array (i.e., a 1×n array), and anend portion of the input optical fiber 22 is secured to the output fibercoupling component 20. Coupling component 20 is used for purposes ofoptical fiber securement, ease of optical fiber handling and precisionoptical fiber placement within spectrometer 10. Coupling component 20may be, for example, a silicon V-groove assembly wherein the opticalfiber 22 is sealed and-aligned in a V-shaped groove formed in a siliconstructure.

Referring to FIG. 3, there is shown a perspective end view of a portionof the spectrometer 10 depicting the one-dimensional sensor array (a 1×4array), and how the single optical fiber 22 is secured to the couplingcomponent 20.

As shown in FIG. 4, the optical sensors 14 and the input fiber 22 aredisposed offset from, but symmetrically about, the optical axis X—X ofthe spectrometer 10 so as to avoid signal interference between apolychromatic optical beam 26 and a substantially monochromatic opticalbeam 24 appearing on or directed to any of the plurality of the opticalsensors 11, or anywhere else. This offset spacing of the optical sensorarray 14 from the coupling component 20 is determined based upon thecharacteristics of diffraction grating assembly 11, the wavelengths ofeach of the substantially monochromatic optical beams 24, and thefocusing power of lens assembly 16.

Lens assembly 16 (FIG. 2) is adapted to collimate substantiallymonochromatic optical beams 24 incident thereon. Lens assembly 16 has arelatively high level of transmission efficiency. The lens assembly mayinclude a plano-convex homogeneous refractive index collimating/focusinglens assembly. Each lens in the lens assembly 16 may utilize arefraction glass material having a high index of refraction to insureefficient optic beam transmissions. It is preferable that lens assembly16 used in modified WDM 505 (ex. spectrometer 10) have telephotocharacteristics. Use of a telephoto lens assembly 16 or a telephoto lensin assembly is advantageous, because lens assembly 16 can be smallerthan would otherwise be required to achieve the same focal length.Alternatively, the lens assembly 16 may include other lens types, lensnumbers, lens configurations and lens compositions. In cases wherediffraction grating assembly 11 is concave or otherwise non-planar, theuse of lens assembly 16 within spectrometer 10 may be unnecessary.

Referring to FIG. 5, there is shown a cross-sectional view of adiffraction grating assembly 11 for use in the present invention.Diffraction grating assembly 11 is a reflective grating for reflectingoptical and/or light rays diffracted therefrom. The grating assembly 11comprises a substrate 11(a) covered by a diffractive surface 11(b). Thediffractive surface 11(b) may be metallic, for example, aluminum orgold. Optionally, an optically transmissive material or coating 13covers diffractive surface 11(b). Substrate 11(a) may be constructedfrom a number of different substances. For example, substrate 11(a) maybe a glass compound. As seen in FIG. 2, substrate 11(a) may have asubstantially planar shape. It is understood, however, that substrate11(a) may alternately include a substantially curved or concave surface(not shown) over which a diffraction grating surface is formed.Generally, as substrate 11(a) is a substantial portion of gratingassembly 11, its thermal properties dominate.

It is understood that although diffraction grating assembly 11 may beassociated with and/or included in passive devices and networks, thatdiffraction grating assembly 11 may also be utilized in devices andnetworks having active components which may perform one or more of avariety of active functions, including optical amplification.

A prism 17 may optionally be disposed between lens assembly 16 anddiffraction grating assembly 11. Prism 17 bends optical signals fromlens assembly 16 towards diffraction grating assembly 11. In doing so,prism 17 allows diffraction grating assembly 11 to be angularly disposedwithin a housing 18 of spectrometer 10, as shown in FIG. 2. Prism 17 maybe in direct contact with material 13 of diffraction grating assembly 11(FIG. 4), or spaced therefrom. It is preferable that prism 17 be spacedfrom lens assembly 16. It is also preferable that prism 17 have a frontangle (θ₁ in FIG. 2), as discussed in more detail below, configured toincrease the angle between reflected substantially monochromatic beamsby anamorphic beam compression. More than one prism 17 can be provideddepending on the configuration of spectrometer 10. It is understood,however, that spectrometer 10 may be utilized without prism 17.

Referring again to FIG. 2, diffraction grating assembly 11, prism 17,lens assembly 16, optical sensors 14, and coupling component 20 withinput fiber 22 are held in relation along axis X—X by a housing 18. Inthe preferred embodiment, housing 18 is a rectangular box structure andis sized to closely receive grating assembly 11, prism 17, and lensassembly 16. It will be readily appreciated that housing 18 can be anyother shape or structure, for example a tube, which conveniently holdsthe components. It will also be appreciated that housing 18 couldalternately be a flat plate which supports the components.

The operation of spectrometer 10 will be described with reference toFIGS. 6A-6B. Spectrometer 10 generally receives a single polychromaticinput signal or beam 26, generates a plurality of individualsubstantially monochromatic signals or beams 24 at different wavelengthsfrom the single polychromatic input signal 26, and those beams areprojected onto sensors 14 to monitor the signal.

A single polychromatic optical input beam 26 is transmitted along andemitted from input optical fiber 22 into the air space between thecoupling component 20 and the lens assembly 16. Within this air space,the polychromatic optical input beam 26 is expanded in diameter (bestseen in FIG. 3) until it becomes incident upon the lens assembly 16. Thelens assembly 16 focuses the polychromatic optical input beam 26 towardsdiffraction grating assembly 11 as polychromatic optical beam 26′ (FIG.6A).

As stated above, diffraction grating assembly 11 operates to angularlydiffract the polychromatic optical beam 26′ into a plurality ofsubstantially monochromatic optical beams 24, with each reflectedsubstantially monochromatic beam 24 being diffracted at a distinctangle, relative to diffraction grating assembly 11, by an amount that isdependent upon the wavelength of the reflected substantiallymonochromatic beam 24. As shown in FIG. 6B, the diffraction gratingassembly 11 reflects the substantially monochromatic signals 24 backtowards the lens assembly 16. The lens assembly 16 collimates theplurality of substantially monochromatic optical input beams 24, andthen transmits each collimated, substantially monochromatic optical beam24′ to the corresponding optical sensor 14. Each substantiallymonochromatic optical beam 24′ becomes incident upon a correspondingoptical sensor 12.

Referring to FIG. 1, the sensor array 535 outputs signals onto a bus 540representative of the measured power for each substantiallymonochromatic optical signal focused on the sensor array 535.Electronics 545 process and/or convert the output signals of the sensorarray 535 and communicate the processed signals to the processor 550 forfurther processing. The processor 550 communicates the further processedsignals to a display driver, which drives a display 560 so that thepower levels for each substantially monochromatic optical signal can bedisplayed for a network operator to visually inspect. The display 560can display the power levels in power (dB) at each wavelength (λ), forexample.

The embodiment of the system 500 as shown is meant only to show thefunctionality of the system 500. It should be understood that thespectrometer 505 may have alternate optical components to perform thesame functions.

Both the mechanical and optical components of spectrometer 10 areaffected by changes in temperature. The materials expand and contractchanging the relative position of components and changing opticalproperties of the system. This negatively impacts the ability ofspectrometer 10 to efficiently demultiplex and monitor an opticalsignal, because it changes the intended path and focus of beams 24 and26. Therefore, to ensure accurate and efficient signal monitoring withspectrometer 10, the thermal effects on components within spectrometer10 are balanced.

To achieve maximum efficiency, each substantially monochromatic beam 24is focused and centered on its respective corresponding optical sensor14. The distance between each beam 24 as it is projected on the end ofits respective corresponding fiber 12 is herein referred to as “lateralspot separation”. Thus, to be centered on sensors 14, the lateral spotseparation of beams 24 must match the spacing of sensors 14.

Lens assembly 16 expands and contracts with temperature changing itsoptical properties. The index of refraction, and thus focal length,changes with temperature, tending to shift the focus, or focal plane,axially and substantially parallel to X—X away from the sensor array 14.Also, as the index of refraction changes, the magnification of beams 24changes, affecting the spot separation. As the magnification increases,the spot separation increases causing beams 24 to expand laterally(lateral expansion) on the sensors 14. As the magnification decreases,the spot separation decreases, also causing movements (lateralcontraction).

The change in optical properties of lens assembly 16 is compounded bythe dimensional change of housing 18 with temperature. Housing 18expands and contracts along axis X—X with temperature, which changes therelative distance between the sensor array 14 and lens assembly 16. Thismoves sensor array 14 axially from the focal plane.

The index of refraction of the air (n_(a)) between grating assembly 11and lens assembly 16, and index of refraction of prism 17 (n_(p)), ifincluded, changes with temperature. This too, affects the averagelateral spot position as refraction of the substantially monochromaticbeams 24 changes.

Substrate 11(a) of grating assembly 11 expands and contracts withchanges in temperature and affects the average lateral spot position. Assubstrate 11(a) expands, the number of diffraction surfaces per unitlength, or grating period (D), decreases. As the grating period (D)decreases, the angle between diffracted substantially monochromaticbeams 24, or angular dispersion, decreases. This can also be quantifiedin terms of linear dispersion, which is the product of angulardispersion and focal length. As the angular and thus linear dispersiondecreases, the lateral spot separation decreases moving laterally onsensor array 14.

Concerning the thermal effects on lens assembly 16, the change ofrefractive index can be quantified in terms of change in optical powerwith temperature. In an embodiment of lens assembly 16 having a singlethin lens element of optical power (φ), the change in power withtemperature (t) is given by: $\begin{matrix}{\frac{\phi}{t} = {{\phi \quad\left\lbrack {\frac{\frac{n}{t}}{n - 1} - {CTE}_{L}} \right\rbrack} = {\phi \quad T}}} & (1)\end{matrix}$

where CTE_(L) is the coefficient of thermal expansion of the lensmaterial and n is the refractive index of the lens in lens assembly 16.

The analysis can be applied to embodiments of lens assembly 16 havingmultiple lens elements. For a system of two lenses with separation d,the total optical power (φ_(T)) is given by:

φ_(T)=φ₁+φ₂ −dφ₁φ₂  (2)

Accounting for change in temperature, equation 2 becomes:$\begin{matrix}{\frac{\phi_{T}}{t} = {\left\lbrack {{T_{1}\phi_{1}} + {T_{2}\phi_{2}} - {{d\left( {CTE}_{D} \right)}T_{1}\phi_{1}T_{2}\phi_{2}}} \right\rbrack \quad \Delta \quad t}} & (3)\end{matrix}$

where CTE_(D) is the coefficient of thermal expansion of a spacerelement (not shown) between lens elements, and where T is the quantityin brackets in equation (1). One skilled in the art will understand thatthis analysis can be expanded to apply to various combinations of thinlenses.

Optionally, the material of lens assembly 16 can be chosen with arelatively low change in refractive index with temperature (dn/dt),herein also referred to as coefficient of refractive index change, tominimize movement in focal plane with temperature.

The material of housing 18 and lens assembly 16 are chosen so that theexpansion or contraction of housing 18 compensates as much as possiblefor axial shift in focal plane. In other words, a length of housing 18between lens assembly 16 and sensor array 14 changes substantially thesame amount as the change in focal length of lens assembly 16 withtemperature. Expressed mathematically:

CTE _(M) L ≅Δz  (4)

where CTE_(M) is the expansion coefficient of housing 18, L is thelength housing 18 between lens assembly 16 and sensor array 14, and Δzis the axial change in position of the focal plane. For a single lens:$\begin{matrix}{{\Delta \quad z} = \frac{1}{T\quad \phi}} & (5)\end{matrix}$

One skilled in the art will understand that this can be expanded toapply to multiple lens systems by applying the same analysis as appliedabove.

Concerning the thermal effects on grating assembly 11, the dispersiveproperties of grating 10 change as substrate 11(a) expands and contractswith temperature. As substrate 11(a) expands and contracts, the angulardispersion, and thus linear dispersion, of reflected substantiallymonochromatic beams 24 changes affecting the lateral spot position. Thechange in dispersion causes a lateral shift in the central wavelength ofthe substantially monochromatic beams 24 as seen by sensors 14,requiring calibration of the sensor array 14 with temperature so thatthe central wavelength is still monitored.

Angular dispersion at Littrow is shown by the following equation:$\begin{matrix}{\frac{\angle}{\lambda} = \frac{m}{2n_{c}D\quad \cos \quad (\theta)}} & (6)\end{matrix}$

where d∠/dλ is the angular dispersion in radians per wavelength, m isthe diffraction order, n_(c) is the refractive index of coating 13, D isthe grating period, and θ is the Littrow angle of the diffractiongrating. As described above, D changes with temperature as substrate11(a) expands and contracts, and thus the angular dispersion changes.

Linear dispersion is the product of the angular dispersion and effectivefocal length, and focal length is the inverse optical power, hence:$\begin{matrix}{{LD} = \frac{\frac{\angle}{\lambda}}{\phi_{T}}} & (7)\end{matrix}$

Therefore, linear dispersion and, correspondingly, lateral spotseparation changes with temperature.

Optionally, to minimize changes in lateral spot separation and lineardispersion resulting from geometrical changes in substrate 11(a), lensassembly 16 can be configured to substantially compensate, and holdlinear dispersion substantially constant, with its change in focallength. Thus, as angular dispersion of grating assembly 11 increases,focal length of lens assembly 16 decreases accordingly, and as angulardispersion decreases, focal length increases to keep linear dispersionsubstantially constant.

Referring to FIG. 7, in practice, substantially monochromatic beams 24are not truly monochromatic, but rather a tight range of wavelengths.Each beam 24 is has a central wavelength 32 which is the representativewavelength to which an optical signal is associated. Each centralwavelength 32 is generally predefined, and may correspond with anindustry standard, such as the standards set by the InternationalTelecommunication Union. As temperature changes the dispersion ofgrating assembly 11, beam 24 is no longer reflected in accordance withthe Littrow and near-Littrow condition discussed above, and the positionof the central wavelength of a beam 24 shifts laterally on sensors 14.

The index of refraction of prism 17, or the air between lens assembly 16and grating assembly 11 if no prism 17 is present, changes withtemperature. This can be balanced with the thermal effects of substrate11(a). The change with temperature of the angular deviation of a beamincident on grating assembly 11 through a prism 17 can be approximatedby: $\begin{matrix}{\frac{\Delta}{t} = {\frac{{- m}\quad \lambda}{\left\lbrack {2n_{p}{D\left\lbrack {1 + {\left( {G + C} \right)\quad \Delta \quad t}} \right\rbrack}^{2}} \right\rbrack}\left( {G + C} \right)}} & (8)\end{matrix}$

where G is the change in refractive index of the prism (n_(p)) overtemperature range Δt, C is the product of the grating substrate 11(a)coefficient of thermal expansion and Δt, and n_(p) is the index ofrefraction of the prism.

To minimize the thermal effects of substrate 11(a), materials of prism17 and diffraction grating substrate 11(a) are chosen so that the changein refractive index of prism 17 with temperature and the coefficient ofthermal expansion of grating substrate 11(a) sum close to zero. Thisminimizes the deviation from Littrow and near-Littrow condition withtemperature, and thus minimizes the lateral shift in center channelwavelength.

It has been found that by choosing the material of prism 17 to have anegative change in index of refraction with temperature approximatelyequal to the coefficient of thermal expansion of substrate 11(a), andpreferably within a magnitude substantially within 30% of thecoefficient of thermal expansion of substrate 11(a), best results areachieved.

When no prism 17 is used, a change in index of refraction of air withtemperature is balanced with the expansion and contraction of substrate11(a). The substrate material is chosen such that its coefficient ofthermal expansion and the change in index of refraction of air withtemperature sum close to zero. In an ideal case, substrate 11(a) has acoefficient of thermal expansion equal to a negative of the change inindex of refraction of air. It has been found that by choosing thematerial of substrate 11(a) to have a coefficient of thermal expansionapproximately between 0.5 PPM/° C. and 1.5 PPM/° C., best results areachieved.

Variations in pressure can affect the path of beams 24 and 26 much inthe same way as temperature discussed above. The most notable effect isthe change in index of refraction of air, especially between gratingassembly 11 and lens assembly 16, with pressure. Prismatic triangularregions of air within spectrometer 10 act as “air prisms” to refractbeams 24 and 26. As pressure varies, the refractive index of air changesand affects how beams 24 and 26 are refracted, thus causing lateralshifts in the position of the center channel wavelength on sensor array14.

Referring to FIG. 8, a prism or prisms 17 may be provided which form aprismatic region of air 34, between prism 17 and lens assembly 16 thatsubstantially balances the refraction of a corresponding prismaticregion of air 36 between prism 17 and diffraction grating 11. An angle(θ_(a)) of air prism 34, measured between prism 17 and lens assembly 16,is approximately equal to an angle (θ_(b)) of air prism 36, measuredbetween prism 17 and grating assembly 11. However, angles θ_(a) andθ_(b) are directed in opposite directions, so that air prisms 34 and 36are opposed as depicted in FIG. 8. This forms opposing air prisms 34 and36, in which the refraction of one balances the refraction of the other.As pressure changes the index of refraction of the air, the change inrefraction of one air prism 34 balances the change in refraction of theother 36.

It is preferable that prism 17 also be configured to create anamorphicbeam compression of substantially monochromatic beams 24 toward sensors14. The anamorphic beam compression decreases the diameter of beams 24,and thus increases the angular deviation between beams. This increasedangular deviation creates additional beam separation at sensors 14 for agiven structure 18 length. Thus, a desired beam separation can beachieved in a shorter overall length of spectrometer 10, enablingspectrometer 10 to be compact.

Referring to FIG. 8, the anamorphic beam compression is accomplished byproviding a prism 17 or prisms configured to maximize the compression.The angular magnification factor is given by: $\begin{matrix}{M = \frac{\sqrt{1 - \left( {\frac{n_{a}}{n_{p}}\quad \sin \quad \left( \theta_{1} \right)} \right)^{2}}}{\cos \quad \left( \theta_{1} \right)}} & (9)\end{matrix}$

where n_(a) is the index of refraction of air and θ₁ is the front prismangle. Thus, θ₁ and n_(p) are optimized, taking into account otherfactors discussed above, to maximize the anamorphic beam compression.

FIG. 9 is a block diagram of a fiber optic network 100 in accordancewith an embodiment of the present invention. The fiber optic network 100provides optical communication between end points 105 a, 105 b, and 105c. Each end point 105 a, 105 b, and 105 c is coupled to a WDM 110 a, 110b, and 110 c, respectively, either optically or electrically. In thecase of an optical coupling, each end point 105 a and 105 ccommunicatesa multiple number of substantially monochromatic optical signals viafiber optic lines 112 a-112 n to the associated WDM 110 a-110 c,respectively. The end point 105 b communicates a multiple number ofsubstantially monochromatic optical signals via fiber optic lines 114a-114 d to/from WDM 110 b, which multiplexes the substantiallymonochromatic optical signals 114 b, 114 d to WDM 110 d along fiberoptic line 116.

The WDMs 110 a and 110 c are coupled via a wavelength add/drop device120 between the fiber optic lines 122 a and 122 c, respectively. Thewavelength add/drop device 120 is, in general terms, a simple form of awavelength router with two input/output (I/O) ports and an additionalthird port wherein substantially monochromatic optical signals are addedto/dropped from the incoming polychromatic optical signal appearing ateither I/O port. Within the wavelength add/drop device 120, a pair ofWDMs 130 a-130 b are utilized to separate a received polychromaticoptical signal into a plurality of substantially monochromatic opticalsignals and communicate one or more of the substantially monochromaticoptical signals to end point 105 b, via the WDM 110 d.

Optical performance monitor (OPM) 135 is further coupled to fiber opticlines 122 a and 122 c. Alternatively, the OPM 135 may be coupled to anindividual fiber optic line. A polychromatic optical signal beingmonitored is tapped or extracted from the fiber optic line 122 a, forexample, via a beam splitter 140 a, as is well known in the art. Thebeam splitter 140 a may tap as little as 1% or less of the power of themonitored polychromatic signal to allow the OPM 135 to properly operateand provide the operator of the fiber optic network 100 valuableoperating information, without substantially affecting the power levelof the monitored polychromatic signal.

It is understood that beam splitter 140 b may be utilized to tap apolychromatic signal appearing on fiber optic line 122 c, and providethe tapped polychromatic signal to WDM 140.

As an example of how the fiber optic network 100 operates, the end point105 a may be located in Boston, the end point 105 b may be located inHartford, and the end point 105 c may be located in New York City. Anetwork service provider in Boston receives communication signals fromlocal towns or cities via a communication system, such as a standardtelephone network. The communication signals, which are destined tolocations south of Boston (i.e., Hartford and New York City), aretime-division multiplexed onto substantially monochromatic opticalsignals and delivered to the WDM 110 a. The WDM 110 a performs a wavedivision multiplexing operation on the substantially monochromaticoptical signals and the resulting polychromatic optical signal istransmitted onto the fiber optic network 100 via the fiber optic line122 a. Upon the polychromatic optical signal reaching a network serviceprovider between Boston and Hartford at add/drop device 120, thepolychromatic optical signal is demultiplexed by the WDM 130 a in thewavelength add/drop device 120. The substantially monochromatic opticalsignals that are destined for New York City may be re-multiplexed by theWDM 130 b and sent to New York City along fiber optic line 122 c. Thesubstantially monochromatic signals destined for Hartford, on the otherhand, may be multiplexed with other substantially monochromatic signals(having different wavelengths) at WDM 110 d and delivered to the endpoint 105 b in Hartford.

In addition, local communication signals originating from Hartford maybe added to either WDM 130 a or 130 b to be transmitted to either Bostonor New York City, respectively, based upon the optical frequency thatthe communication signals are placed. The substantially monochromaticoptical signals are multiplexed by WDM 130 b into a polychromaticoptical signal and demultiplexed by WDM 110 c in New York City. Itshould be understood that the fiber optic lines (e.g., 112 a, 122 a,116, 122 c) are bidirectional such that optical communication can beperformed in either direction. The network service provider associatedwith add/drop device 120 additionally may monitor the system performance(e.g., channel power) using the OPM 135 to ensure system quality.

The present invention is not to be limited in scope by the specificembodiments described herein. Indeed, various modifications of thepresent invention, in addition to those described herein, will beapparent to those of skill in the art from the foregoing description andaccompanying drawings. Thus, such modifications are intended to fallwithin the scope of the appended claims.

What is claimed is:
 1. A device for monitoring wavelength divisionmultiplexed optical signals, comprising: a structure for supportingcomponents of the device; an optical component supported at one end ofthe structure for transmitting the optical signals; a diffractiongrating supported at an opposing end of the structure for diffractingthe optical signals from the optical component; an optical sensorsupported in relation to the diffraction grating by the structure formonitoring the optical signals; a lens assembly supported by thestructure and disposed between the optical sensor and the diffractiongrating, the lens assembly having a focal length for focusing theoptical signals in relation to the optical sensor; and wherein thecoefficient of thermal expansion of the diffraction grating is a valuechosen to be approximately equal to a negative of a change of index ofrefraction of air with temperature.
 2. The device of claim 1 wherein thediffraction grating has a coefficient of thermal expansion, ofapproximately 0.5 PPM/degree Celsius to 1.5 PPM/degree Celsius.
 3. Thedevice of claim 1 wherein the lens assembly is constructed of a materialchosen to minimize its variance in focal length over temperature.
 4. Thedevice of claim 1 wherein the lens assembly comprises a telephoto lens.5. The device of claim 1 wherein a coefficient of thermal expansion ofthe structure and a change in index of refraction with temperature ofthe lens assembly are values selected so that a length of the structurechanges substantially proportionally with the focal length of the lensassembly in response to temperature changes, whereby the lens assemblyremains substantially focused in relation to the optical sensor.
 6. Anoptical network having a device for monitoring wavelength divisionmultiplexed optical signals, comprising: a structure.for supportingcomponents of the device; an optical component supported at one end ofthe structure for transmitting the optical signals; a diffractiongrating supported at an opposing end of the structure for diffractingthe optical signals from the optical component; an optical sensorsupported in relation to the diffraction grating by the structure formonitoring the optical signals; a lens assembly supported by thestructure and disposed between the optical sensor and the diffractiongrating, the lens assembly having a focal length for focusing theoptical signals in relation to the optical sensor; and wherein thecoefficient of thermal expansion of the diffraction grating is a valuechosen to be approximately equal to a negative of a change of index ofrefraction of air with temperature.
 7. The network of claim 6 whereinthe diffraction grating has a coefficient of thermal expansion ofapproximately 0.5 PPM/degree Celsius to 1.5 PPM/degree Celsius.
 8. Thenetwork of claim 6 wherein the lens assembly is constructed of amaterial chosen to minimize its variance in focal length overtemperature.
 9. The network of claim 6 wherein the lens assemblycomprises a telephoto lens.
 10. The network of claim 6 wherein acoefficient of thermal expansion of the structure and a change in indexof refraction with temperature of the lens assembly are values selectedso that a length of the structure changes substantially proportionallywith the focal length of the lens assembly in response to temperaturechanges, whereby the lens assembly remains substantially focused inrelation to the optical sensor.